As I've been messing with my brakes it got me thinking. How quickly does brake fluid absorb moisture? I personally use a new sealed bottle whenever I use any and after a bit of Googling I kind of found an answer in that the fluid will start to absorb moisture immediately that it's exposed to atmosphere but I suppose I'm asking, to what extent?
How much moisture per litre, say, would make the fluid unusable and how quickly would it take exposed fluid to reach that point?
Just wondering.

depends on the fluid,

though the recommendation on the standard fluids are about 3-4 years, however, there are many cars where the fluid has been in well over a decade, and they still 'work'

I have a 1998 Sprinter which has never had a fluid change, I had a stuck caliper on the front which got the disc literally "red hot", proper glowing, I still had a reasonably firm pedal. Conversely a slightly dragging rear caliper on my 2006 Seat Altea, bit of a smell of burning pad but no glowing disc, had the pedal going to the floor.

I can't explain why the outcome was so different, presumably the Altea fluid was quite contaminated for some reason, I changed the Altea brake fluid.

Some reservoirs have a rubber membrane between the fluid and the air gap so air is not in contact with the fluid.

Other reservoirs just have a pin hole in the cap.



HNY
Paul G

I have a device that samples the brake fluid and gives a reading indicating replace or otherwise. It's a cheap device but helps no end in decision making.

Thanks for the replies. Does anyone have any statistics? Was just interested.

Pretty much any car with 4 +yr old fluid in will fail the test kit I use at work . Unless you do some pretty full on driving on the road you’d be hard pressed to feel any improvement after having the old stuff flushed .

This its not an mot test kit is it?

As yours and others mention, i know and own(ed) cars that have never had it fully replaced

Just part of the service checks , dip test and refractometer . I’m still working my way through a 5 ltr ate blue fluid I got 10yrs ago .

I change DoT 4 brake fluid every 2 years.

It is also a good chance also to check over the condition of the brake system and adjust the rear drums (on my Austin Healey Sprite) - sorting any damaged/corroded pipes, worn components, as well as having new fluid without water in it and system bled from air (also do hydraulic clutch). Apart from compression issues with water and air, I am not keen on any chance for increased internal corrosion in the calliper pistons, brake master cylinder and wheel cylinders that could contribute to dragging or sticking brakes (and more time, hassle and money for repairs)

Brake fluid is like tyres, no point taking a risk with poor condition and low quality. Including on daily use cars as well as seldom used sports cars, kit/self build cars and classics.

Boring and conservative, I know, and contributing to the brake fluid manufacturer's profits, but there you go.

Cheers
Mike

[Edited on 1/1/18 by Mike Wood]

[Edited on 1/1/18 by Mike Wood]

[Edited on 1/1/18 by Mike Wood]

[Edited on 1/1/18 by Mike Wood]

[Edited on 1/1/18 by Mike Wood]